Composite elements which are made up of two outer metallic layers and an intermediate layer of a polymer foam are increasingly being used as facade elements for buildings, in particular in the commercial sector, such as warehouses or industrial buildings, for instance workshops, sports halls, department stores or assembly shops. The composite elements used in facade construction are often also referred to as sandwich elements or sandwich panels.
The use of composite elements allows a rapid construction of the respective building, since only one supporting structure of metal, concrete or wood, to which the composite elements are subsequently attached, has to be built. A further advantage of composite elements is their good thermal insulation, which derives from the structure comprising the inner core of polymer foam.
Various methods which can be used to produce composite elements usable as facade elements are known, for instance, from WO-A 2009/077490, WO-A 2010/060864 or WO-A 2012/093129. In these, a fundamental point is in each case the good adhesion of the layer of polymer foam to the respective cover layers of metal in order to prevent the metallic cover layers from becoming detached from the polymer foam core, thereby resulting in undesirable damage to the composite elements.
A further fundamental aspect is the fastening of the composite elements to the supporting structure. This fastening of the composite elements is generally effected by a screw connection. Thus in “Bauen mit Stahl, Stahlbau Arbeitshilfe No. 46, Sandwichelemente”, different variants for the assembly of composite elements used as wall elements or roof elements are described. In this case, in particular a screw connection variant in which the screws used for the fastening are not visible from the outside has won approval. The composite elements are here equipped with tongue and groove and in the region of the tongue the composite elements are screwed to the supporting structure. The next composite element is slid with the groove over the tongue and the screws are hereby concealed. The positions and the nature of the screw connection are generally specified and described in the technical documentation of the manufacturer of the composite elements. In contrast hereto, the composite elements of cold stores are generally screwed visibly along the individual beams of the supporting structure.
In the case of screw connections of the composite elements, in particular of the kind used for the construction of cold stores, a failure of the composite elements through formation of a kink in the outer metallic cover layer has been observed. This kink is attributable to different thermal expansions, in particular during solar radiation onto the outer side of the cold store, since the composite elements on the outer side can heat up strongly under solar radiation. On the other side, temperatures within the range from −35 to −20° C. usually prevail in the interior, so that, across the composite elements, a temperature difference of up to 80° C. can arise. The outer cover layer expands very much more strongly, which in the worst case results in total failure as a result of buckling of the metallic outer cover layer. The corresponding mechanisms have been described, for example, by Jörg Lange, Ren{tilde over (e)} Mertens, Sandwich am Bau, Thema FORSCHUNG 1/2006, pages 30 to 34.
In order to counter the kinking due to large temperature differences on the inner side and outer side of the composite elements, for the construction of low-temperature stores in silo construction it has been proposed in DE-A 38 34 612 to use clamps to fasten the composite elements to the supporting structure, which clamps enable displacement. Clamps for fastening natural stone slabs for facings are known from DE-A 100 28 750. However, this type of fastening is used exclusively for low-temperature stores or cold stores, while an application for other buildings is not known.
In contrast to the known formation of a kink in the outer metallic cover layer of the composite element, it has now been shown that, in screwed composite elements having dark outer surfaces, an irreversible, wavy deformation can arise. This deformation can in particular be observed when the facade elements have been mounted at low temperatures and have a plane or almost plane surface. Moreover, this phenomenon also occurs in particular in composite elements which have a length of at least 3 m. As a result of the wavy deformation of the outer surface, the core of polymer foam can locally tear and, as a result of reversible cooling and heating, sizeable blisters are formed. The heating here occurs, for example, through solar radiation, and at night, or when there is no solar radiation, for example on cloudy days, the surface cools down again.